The reliability of distribution transformers under lightning conditions has been a long standing subject of concern for both the users of distribution transformers and distribution transformer manufacturers. Lightning induced current surges and induced voltage surges from lightning related phenomena can cause winding failures in the high voltage windings of a single phase distribution transformer. As is set forth in "Low-Voltage-Side Current-Surge Phenomena In Single-Phase Distribution Transformer Systems" IEEE/PES T and D Conference and Exposition, Paper 86T&D553-2, September 1986, R. C. Dugan and S. D. Smith:
1) customer load is more susceptible to damage due to lightning-induced voltages under light load conditions;
2) at a given loading, systems with interlaced transformers cause higher lightning-induced voltages across customer loads than appear in systems with non-interlaced transformers; and
3) applying arresters across the non-interlaced low-voltage winding will increase the lightning-induced voltages across the customer load to nearly the same level that occurs with an interlaced transformer.
These findings were made during a comprehensive study which demonstrated the significance of system parameters in lightning-induced surges in distribution transformers. Interlaced windings can in fact make a distribution transformer less susceptible to certain failures that can be induced by the secondary side current surges created by lightning strokes to either the primary system or the secondary system. However, the initial manufacturing cost of interlaced windings as well as future cost of losses of single phase distribution transformers incorporating interlaced windings are significantly greater than compared to non-interlaced low-voltage windings. This difference could amount to as much as one millions dollars per year in total owning costs for a pole-mounted distribution transformer.
In an attempt to overcome the high cost associated with these interlaced windings in distribution transformers, lightning arresters have been applied across the two halves of the low-voltage windings of a non-interlaced distribution transformer in order to prevent the surge currents from entering the low-voltage windings. Moreover, it has been found that the use of internally applied MOV arresters in combination with externally applied spark gaps do in fact protect the secondary side of non-interlaced distribution transformers from lightning-induced surge currents.
However, as with interlaced windings, internally applied MOV arresters are expensive and therefore, add significantly to the manufacturing costs, and subsequently to the owning costs of distribution transformers. Additionally, and more importantly, externally applied spark gaps applied at the X1 and X3 terminals of pole-mounted distribution transformers, while being cost effective, are relatively unreliable and could result in the systems inability to prevent surge current from entering the low-voltage windings of the distribution transformer. Externally mounted spark gaps which are applied at the X1 and X3 bushings of distribution transformers must be set during or shortly after the installation of the transformer, and if the externally mounted spark gap's air gap is not properly set or damaged due to handling of the transformers, the externally applied spark gap could be rendered ineffective. Moreover, because the externally mounted spark gaps are in fact mounted on that portion of the X1 and X3 terminals which extend outside the tank of the pole-mounted distribution transformer, these externally mounted spark gaps will be subjected to adverse environmental conditions which could readily render the externally mounted spark gap ineffective. This would then allow lightning-induced current surges to enter the low-voltage windings thereby possibly resulting in the failure of the primary winding of the distribution transformer.
Therefore, in view of the foregoing there is clearly a need for both an economical and reliable mechanism for bypassing the secondary side surge component of lightning-induced surges and induced voltage surges from lightning related phenomena around the low-voltage windings in order to prevent failures in the primary windings of distribution transformers. Moreover, while not only being reliable, such a mechanism must be capable of safely operating under severe transformer operating conditions.